The sample preparation process has a direct impact on accuracy, precision, and quantitation limits and is often the rate determining step for many analytical methods. For example, in environmental analysis, the analytes are present in only trace quantities in solution, and dilute aqueous samples must be processed in order to isolate and concentrate analytes from the sample matrix and provide a suitable sample extract for instrumental analysis. Sample preparation is arguably the most important step in the analytical process. Analytical chemists therefore continue to search for sample preparation procedures that are faster, easier, safer, and less expensive to perform yet provide accurate and precise data with reasonable quantitation limits.
Solid phase extraction (SPE) techniques have replaced many methods for the determination of organic analytes in aqueous samples. SPE methods use solid sorbents which are typically packed into disposable plastic or glass cartridges. When a liquid sample containing analytes is passed through a bed of solid phase sorbent, the analytes are retained on the sorbent and thereby separated from the liquid. Subsequently, the analytes can be eluted from the sorbent by using a relatively small volume of a suitable solvent, whereby a relatively concentrated solution of the analytes can be obtained, and the resulting solution can be subjected to suitable instrumental analysis techniques in order to identify and/or quantitate the analytes that were present, possibly in trace amounts, in the original liquid sample.
Conducting the SPE sample preparation typically involves at least three steps: (1) eluting the liquid sample through the bed of solid phase sorbent to extract the analytes; (2) drying the sorbent by passing nitrogen, or other inert gas, or air through the sorbent bed; and (3) eluting the analytes from the sorbent bed using a suitable solvent.
The sorbent must be completely dried before the samples are eluted, typically with an organic solvent, to obtain the solution of analytes for instrumental analysis. This is typically achieved by passing an air or inert gas flow through the sorbent bed. However, particularly when aqueous samples are being analyzed, drying takes a long time. For example, with a typical SPE cartridge, it generally takes about five to twenty minutes or longer to dry the cartridge completely. During the drying process a large amount of air or nitrogen is passed through the sorbent.
Use of nitrogen, or another bottled inert gas, is one option for drying the sorbent bed. However, providing the supply of pressurized inert gas such as nitrogen is very inconvenient and expensive. Typically, the gases are supplied at high pressure—over 2000 p.s.i.—in large, heavy, metal cylinders weighing over 150 pounds that can be difficult and hazardous to move and store. Laboratories typically must pay both to rent the cylinder and purchase the gas. Because of the large amount of gas often required to dry the sorbent bed, the cylinders are quickly exhausted of nitrogen and must be refilled, which is both inconvenient and expensive. Unless expensive, high quality, “purity assured” gas is used, the cylinders may be contaminated, and the contaminants from the gas may be adsorbed onto the solid phase sorbent and interfere with the analysis. Even if the highest quality, most expensive, inert gas is used, another problem that must be overcome is potential contamination of laboratory equipment; for example, in the tubing connecting the inert gas supply to the SPE cartridge. In addition, inert gases such as nitrogen are asphyxiant, so care must be taken to provide adequate ventilation if large amounts of the inert gas are being released.
On the other hand, although using air to dry the sorbent is inexpensive, convenient, and safe, a significant problem occurs because of the large volume of air required by the drying process. Contaminants present in the air used to dry the sorbent can be adsorbed onto the sorbent, and interfere with the analysis. Since the analytes may be present in only trace quantities in the liquid samples being analyzed, contaminants adsorbed from the air can interfere significantly with the analysis. This may be exacerbated in a laboratory setting where chemicals, possibly including standard samples of the analytes of interest, may be being used, and therefore may be present in the laboratory air.
An apparatus and method is therefore needed to enable SPE sorbents to be inexpensively and conveniently dried, which avoid the expense, inconvenience and hazards associated with using bottled inert gas, but also avoid the problems with contamination by airborne contaminants that can be observed when air is used to dry the sorbents.